Known metallic universal joint prostheses for hip joints have a complete, spherical head and a bent or curved stem or shaft connected rigidly thereto, wherein the head and the stem or shaft have essentially full cross-sections, i.e. are free of holes. In order to attach such a prosthesis to a bone, i.e. the femur, the natural head and a part of the neck of the femur are cut off and a hole is drilled in the shaft of the femur so that a great deal of bone material has to be removed and many blood vessels are destroyed. In addition, the bent or curved metallic prosthesis stem or shaft which is usually cemented into the femur shaft has a much greater bending strength than a natural femur. This has the disadvantage that, under loads, regions of the femur which are adjacent to the stem or shaft and possibly also other parts of the body frequently experience much stronger and/or weaker localized stresses than in the case of corresponding loads on a completely natural femur, and that consequently the long-term stability of the connection between the prosthesis and the femur may be impaired.
Universal joint prostheses disclosed in U.S. Pat. No. 4,328,593 and U.S. Pat. No. 4,332,036 have a cap and a stem which consists of a cylindrical sleeve having a perforated jacket. The cap has a calotte-shaped, smooth outer surface and an inner surface having projections and recesses distributed along the circumference. These known prostheses consist, for example, of titanium, the inner surface of the cap and the sleeve having a thin porous titanium coating applied by a plasma spraying method.
Compared with the prostheses having a solid head and a solid, bent or curved stem or shaft cemented in the bone, these prostheses having a cap and a sleeve of titanium have the advantages that only relatively little bone material must be removed for attaching the prosthesis. Furthermore, the bone material can readily grow to the inner surface of the cap and also unite with the perforated sleeve, so that the prostheses disclosed in the cited U.S. Pat. Nos. 4,328,593 and 4,332,036 are connected relatively firmly to bones without cement. In the case of extremely severe and/or quickly changing loads, however, there is in fact a certain danger that the connection between the inner surface of the cap and the bone will experience localized loosening.
Since the sleeve of the prostheses disclosed in the cited U.S. Pat. Nos. 4,328,593 and 4,332,036 has a relatively large diameter, it is furthermore difficult in certain cases to cut the annular hole which serves for receiving the sleeve into the bone without damaging blood vessels or causing other injuries during the operation. Another certain disadvantage of these known prostheses is that the calotte-shaped outer surface of a titanium cap--even when it is polished smooth--gives rise to relatively great fiction in the case of movements in an artificial or natural socket and causes relatively pronounced wear of the socket and/or is itself worn away.